(a) Nonperceptible electrical current.
(b) Perceptible
(perhaps
painful)
currents
below
the
let-go-
threshold.
(c) Currents above the let-go-threshold.
(d) Currents which cause ventricular fibrillation (discoordinated
heart action).
The currents passing through the human body which cause the various
physiological responses are provided in table 4.
The thresholds of these
effects are influenced by the current path through the body.
The actual
magnitude of current flow, resulting from accidental contact of live
electrical circuit components, is determined by skin contact resistance.
Current magnitude is determined by Ohm's Law; i.e., I=E/R, where I = current
through body, E = applied voltage to body contacts, and R = total resistance
to current flow. This resistance is the sum of two resistances in series,
contact resistance, and internal body resistance. Internal body resistance
is a constant value, varying between 200 to 500 ohms depending on current
path, whereas contact resistance may vary over several orders of magnitude
between 1 and 100 ohms.
High voltages (600 volts cited in one reference)
may cause breakdown of contact resistance, in which case contact resistance
becomes negligible.
Table 4.
Quantitative Effects of Electric Current on Humans
78
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